The telecommunications Plesiochronous Digital Hierarchy (PDH) is widely used to transmit digital information (e.g. digitised telephone calls) along telephone cables at any of a number of standardised data rates. For rates of 44.768 MHz and below a ternary (three-level) signal waveform is used. In this waveform a logic 1 symbol is indicated by the presence of a pulse (relative to a predefined timebase) and a logic 0 symbol is indicated by the absence of a pulse. In general successive logic 1 symbols are represented by respective pulses of opposite polarity to one another. Thus a data signal is made up of positive and negative pulses indicating logic 1 symbols and gaps indicating logic 0 symbols.
PDH transmitter circuits conforming to standards issued by the European Telecommunications Standards Institute (ETSI) generate relatively rectangular positive and negative pulses. However, if the received signal has propagated through a long length of coaxial or twisted pair cable, it will be distorted by the frequency-dependent loss characteristic of the cable. This loss characteristic in the case of twisted pair cable is typically represented by the expressionLoss=(P×√{square root over (f)}+Q×f)×lwhere P and Q are constants dependent upon properties of the cable such as size, twists per unit length and conductor thickness, f is the signal frequency and l is the cable length.
Purely resistive attenuation, or flat loss, reduces the amplitude of the received signal without affecting the pulse shape. Cable attenuation however not only reduces the amplitude of the signal but also affects the shape of the received pulses. In general therefore the amplitude of the received signal is a function of the amplitude of the transmitted signal, the flat loss and the cable attenuation.
Signals transmitted at the ETSI E1 rate of 2.048 Mbits/second are frequently carried over twisted pair cable, so the pulse shape at the receiving end of the cable can be severely attenuated and distorted owing to this frequency dependent loss. Circuits which receive E1 signals are therefore designed to restore the amplitude and pulse-shape before attempting to identify logic 1 pulses in the signal. This restoration process is known as equalisation and the amount of equalisation required to restore a signal depends upon the length of cable transited by the signal. PDH receiver circuits known as line interface units (LIUs) are commercially available in integrated circuit form to perform equalisation together with clock recovery and data restoration.
Instruments designed to test the operation of PDH equipment by examining the signals they generate and receive must be able to differentiate between bit errors caused by the equipment under test and errors due to the received signal amplitude being too small. In the latter case, the test equipment should report a LOSS OF SIGNAL (LOS) condition (rather than a fault in the unit under test), ideally at a signal level just above that at which the attenuation of the signal would start to cause errors. In practice LOS is reported by a test instrument when the amplitude of the monitored signal falls below a predetermined threshold. This LOS threshold is usually determined during manufacturing calibration of an instrument by reducing the amplitude of a signal input to the instrument, as if the signal were affected by flat loss, to an ‘onset of errors’ level where data errors begin to occur, and then setting the LOS threshold just above this onset of errors level. For example, if errors begin to occur when a signal is attenuated by 10 dB the LOS threshold would be set at the 9 dB attenuation level.
PDH test equipment conveniently uses a receiver LIU to provide equalisation and thus restore pulses distorted by cable attenuation. An LIU is capable of restoring a signal which has been reduced in amplitude by cable attenuation to a level much smaller than that which would result in errors if the attenuation were caused by flat loss. Whereas a signal affected only by flat loss may start to show attenuation-induced errors at a received amplitude 7 dB below nominal, a signal which has travelled along the maximum allowable length of twisted-pair cable and arrives at the receiver LIU with an attenuation of for example 14 dB may well produce no errors in the data error checking circuits after equalisation. The actual LOS level used in a measurement must therefore be recalibrated to take account of the amount of cable attenuation suffered by the incoming signal, so this cable attenuation must in turn be ascertained.